1. Field of the Invention
The present invention relates to a method of forming a complementary metal-oxide-semiconductor (CMOS) image sensor, and more particularly, to a method of forming an improved photo-diode sensor of an image sensor.
2. Description of the Prior Art
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a top-view diagram of an prior art image sensor on a semiconductor wafer 10. FIG. 2 is a cross sectional diagram along line 2--2 of the semiconductor wafer 10 shown in FIG. 1. A CMOS image sensor comprises three NMOS transistors used as a reset MOS, a current source follower and a raw selector, respectively, and a photo-diode sensor for sensing the photo-intensity. As shown in FIG. 1 and FIG. 2, the image sensor is formed on a semiconductor wafer 10 which is covered with a positive-type (P-type) substrate 12. The surface of the semiconductor wafer 10 comprises a negative-channel (N-channel) area 30 for forming three negative-type MOS (NMOS) transistors and a sensor area 32 for forming a photo-diode sensor. The NMOS transistors in the N-channel area 30 are formed by using a conventional lightly doped drain (LDD) process. In the conventional LDD process, first, a P-type well 13, N-type well, at least one field oxide layer 14 and a gate electrode 16 comprising a conductive layer 18 are formed sequentially. Then, a LDD layers 20 is formed by using an ion-implantation process. Next, spacers 22 are formed. Finally, a heavy doped drain (HDD) layer 24 is formed by using another ion-implantation process with arsenic (As) as the major dopant at a dopant concentration at about 10.sup.19 to 10.sup.20 cm.sup.-3 so as to complete the NMOS transistors. At the same time, a HDD layer 24 is also formed in the sensor area 32 wherein the HDD layer 24 and the P-type well 13 form a PN junction that functions as a photo-diode sensor.
The HDD layer 24 of the photo-diode sensor and the HDD layer 24 of the NMOS transistors are formed at the same time. That is the HDD layer 24 with heavy As forms one side of the photo-diode sensor. In results, there are many shortcomings in the characteristics of the photo-diode sensor, for examples:
1. The atomic size off As is bigger and the dopant concentration is heavier, so the silicon crystal structure within the surface of the sensor area is destroyed severely which leads to an increase in the leakage current of the PN junction, and an increase in the leakage current (also called dark current) of the PN junction when the photo-diode sensor does not accept light. This will increase the noise of the image sensor and decrease the resolution of the image sensor. PA1 2. The width of the depletion region of the PN junction formed with the HDD layer 24 is narrower which leads to a smaller real active region of the sensor area. This will decrease the leakage current (also called photo-current) of the PN junction when the photo-diode sensor accepts light. Therefore, the sensed photo-signal is smaller and the photo-sensibility is worse. PA1 3. The lifetime of charge carriers (also called photo-charges) simulated in the HDD layer 24 caused by accepting light will decay severely. That is the recombination rate of the photo-charges increases as the dopant concentration of the HDD layer 24 increases which leads to a decrease in the generated current after the acceptance of light. Especially, when a short-wave light such as cyan-light irradiates the semiconductor wafer, the skin depth of the semiconductor wafer is shallower. So the sensitivity to the short-wave light will be much worse as the recombination rate of the photo-charges increases. PA1 forming at least one gate electrode in the N-channel area; PA1 performing a second ion-implantation process to form a heavy doped drain (HDD) layer in another predetermined area on the surface of the substrate in the N-channel area next to the LDD; and PA1 wherein the dopant concentration of the HDD layer is heavier than that of the LDD layer, and the doped layer in the sensor area and the P-type substrate under the doped layer form a PN junction that functions as a photo-diode sensor.